1. Field of the Invention
The present invention relates to packet forwarding apparatus (equipment) for performing the transmission of telecommunication traffic packets and further relates to a communication network having a plurality of terminals and a transport network for interconnecting the plurality of terminals.
2. Description of the Related Art
A packet switching method for controlling the transmission data in packets is broadly employed in data communication networks or local area networks whose number of nodes is large and whose bandwidth necessary for each node varies during a short period of time. This is because the packet switching method can effectively utilize a transport bandwidth as compared with a circuit switching method. The circuit switching method is a switching method for controlling the transmission data in units having a multiplexed and determined transport rate by space division multiplexing, time division multiplexing, frequency division multiplexing and code division multiplexing. It is difficult for a network using the packet switching method (hereinafter, a packet switched network) to always maintain a transport bandwidth for a designated route and monitor a transport status as in a network using the circuit switching method (hereinafter, a circuit switched network) and a delay for every node on a traffic path is large because switching is performed by the store and forward method. Therefore, in a packet switched network, it is difficult to construct a highly reliable network as compared with a circuit switched network.
On the other hand, the circuit switching method is always employed for a backbone network having a relatively small number of nodes and requiring a high reliability. The demand of a bandwidth for the backbone network has been increasing these days and a switch having a large switching unit capacity is required for the main network. The reason for that demand is that (1) it is preferable to reduce the number of switches, each of which operates for a high speed recovery from a failure, and (2) it is preferable to minimize the size and cost of the hardware by restricting the number of the circuits being applied to backbone nodes.
Most actual large scale datagram networks have a multi-layer structure where a packet switched network employs the backbone network as a transport medium. In such a communication network, the packet switched network overlays the circuit switched network. The problem with having the data network overlaying the packet switched network on the circuit switched network is that there is a brief disconnection caused by an instantaneous cut off through switching by the circuit switched network.
In the present packet forwarding protocol, the receipt confirmation of a packet is performed by a receiver side that transmits a receipt notification message (ACK packet) to a transmitter side. When the packet is not received, the receiver side outputs a resending request to the transmitter side by a non-receipt notification message (NAK packet).
In the current operated packet switched network, the data communication service is remedied by the data recovery through the data resending rather than the data switching without data loss. When a circuit having a large capacity halts the data transmission service temporarily, an enormous amount of control packets are generated for resending non-received packets and the utilization efficiency of the network bandwidth deteriorates. Since resending by the packet forwarding protocol takes time to transmit the control packet therebetween, failures may sometimes occur for the service which requires the real time responsibility for transmitting voice data and for the service necessary to keep an order of arrival packets.
In the research level, there is an attempt to perform the switching without disconnection on the packet switched network. In particular, in an asynchronous transfer mode (ATM) network, there is proposed a method for switching without disconnection in which data are always transmitted to pluralities of routes. At the time of switching, a selector on the receiver side changes the connection destination after the selector on the receiver side synchronizes a signal from the switching source route with a signal from the switching destination route.
There is a problem with the above proposal in that it is difficult to perform the synchronization of data coming from pluralities of routes. In order to solve this problem, there is proposed a method for the insertion of a cell that is marked, as described in a Japanese laid open patent No. 7-264210, and/or for transmitting a dummy cell until when there is no useful information on the both switching source and destination routes, as described in a Japanese laid open patent No. 8-139732.
The switching without disconnection in the circuit switched network is studied in the network of SONET/SDH standard, as described in Kawase et. al., xe2x80x9cHitless Frame Switching Scheme for SDH Networks,xe2x80x9d (The institute of Electronics, Information and Communication Engineers, Vol. J78-BI No.12, pp. 764-772, 1995). Because the network of SONET/SDH standard is a synchronous network, it is necessary to perform accurate control as compared with the switching without disconnection in the packet switched network since the synchronization within a byte must be implemented for matching the signal phase of data coming from the pluralities of routes at the time of switching.
Although the conventional SONET/SDH method has been broadly employed in the backbone network, an installation of an optical switching method for performing the switching by using an optical switch to the backbone network is studied. The optical switching method is by switching with a light without an optical-electrical conversion of the main signal at the time of switching an optical transport route.
The feature of the optical switching method is that a switching unit capacity can be increased as compared with an electrical switching in the SONET/SDH method. However, it is difficult for an optical signal to store the data signal into a storage. Thereby, it is still difficult for an optical switching network to perform the switching without disconnection by using a buffer memory for storing a data signal.
An object of the present invention is to reduce an amount of packet data loss when the state transition or state change occur in association with the brief disconnection in the circuit switched network (e.g., brief disconnection for the recovery from the transport route failure) upon which the packet switched network is overlaid.
The packet switched network is configured by connecting a plurality of packet forwarding apparatus with a transport link. In the packet switched network overlaying the circuit switched network, a circuit corresponds to a transport link. The packet forwarding apparatus of the present invention is assumed to include a storing means. Although most packet forwarding apparatus include the storing means for performing a switching by a store-and-forward method, it is preferred to add a storing capacity when the storing means does not have a capacity sufficient for an embodiment of the present invention. It is still preferred to insert or add a storing means into the packet forwarding apparatus for applying the embodiment of the present invention to a packet forwarding apparatus not including the storing means. When a circuit connected with the packet forwarding apparatus undergoes a brief disconnection caused by switching, after a packet data transmitted to the circuit only during the period of the brief disconnection is stored into the storing means, if a packet transmission to the circuit is halted, the data loss on the packet switched network can be avoided. The packet transmission halt is performed by receiving a brief disconnection begin notification from the circuit switched network. When the circuit switched network is going into the state transition, the brief disconnection begin notification is produced by an administrator, an administration apparatus of the circuit switched network or a transport supervisory apparatus and is transmitted to the packet forwarding apparatus through the supervisory channel prepared for each circuit or the supervisory network being prepared. When the period requiring the brief disconnection has finished, the circuit switched network sends a brief disconnection end notification to the packet forwarding apparatus and the packet forwarding apparatus that receives the brief disconnection end notification restarts the data transmission stored in the storing means.
FIG. 1 shows an example network configuration that realizes the aforementioned function. The network as in FIG. 1 has packet forwarding apparatus 20-1 through 20-5 and a circuit switched network for providing a connection between the packet forwarding apparatus, and the circuit switched network includes circuit switching apparatus 10-1 through 10-5 and transport link 30 for connecting between the circuit switching apparatus. When there occurs the necessity to change a transport route A between packet forwarding apparatus 20-1 and 20-2 to a transport route B between packet forwarding apparatus 20-1 and 20-3, the circuit switched network sends a brief disconnection begin notification to a packet forwarding apparatus 20-1 on the transmitter side, halts a packet transmission from the packet forwarding apparatus 20-1 and performs the switching by using each circuit switching apparatus. After the switching is completed, the circuit switched network sends a brief disconnection end notification to the packet forwarding apparatus 20-1 and the transmission of the packet from the packet forwarding apparatus 20-1 is restarted.
For another method, at the time of the brief disconnection during the service of the circuit switched network, the service failure in the packet switched network can be avoided by providing packet forwarding apparatus on the transmitter side and on the receiver side useful for temporarily changing a route of a packet data being transmitted via the circuit switched network to a redundant circuit useful for rerouting.
FIG. 2 shows an example configuration of the circuit switched network embodying the aforementioned method. The network as in FIG. 2 includes packet forwarding apparatus 20-1 through 20-5 and a circuit switched network for providing a connection between the packet forwarding apparatus and the circuit switched network further includes circuit switching apparatus 10-1 through 10-5 and transport links 30 for connecting between the circuit switching apparatus.
Circuit switching apparatus 10-1 and 10-2 provide both a port 11-1 connected with routes A or B as a transport route between the packet forwarding apparatus 20-1 and 20-2 and a port 11-2 connected with a redundant route C different from any one of the above routes. In the circuit switched network of FIG. 2, at the time of switching the circuit switched network, a brief disconnection begin notification is transmitted from one of the circuit switching apparatus in the circuit switched network to a packet forwarding apparatus on the transmitter side, a packet forwarding apparatus on the receiver side or both of them. When either one of the packet forwarding apparatus receives the brief disconnection begin notification, the packet forwarding apparatus on the transmitter and receiver sides operate together and switch a port sending and receiving a packet, which is useful for the packet transmission, with a port connected with a redundant route only during the circuit switching operation.